1) Field of the Invention
The present invention relates to a welding method for forming a build-up layer on base materials having a directionally solidified crystal, such as a single crystal material or a unidirectionally solidified crystal material.
2) Description of the Related Art
A centrifugal force works on buckets of a turbine, a jet engine and the like, which rotate at high speed; therefore, a unidirectionally solidified crystal bucket of which crystal orientation is oriented to a direction of a centrifugal force or a single crystal bucket that is wholly formed by a single crystal material are used for the above-mentioned buckets. Cracks or fractures occur in these directionally solidified crystal buckets through use. And casting defects occur during a manufacturing process.
Conventionally, these defects that occurred in the buckets have been repaired through brazing or welding. There is no problem in repairing ordinary casting buckets through the conventional repairing methods. However, if the conventional repairing methods such as brazing or welding are applied to the directionally solidified crystal buckets such as the single crystal bucket or the unidirectionally solidified crystal bucket, strength of the repaired part problematically lowers due to the repaired part does not become the directionally solidified crystal.
To solve the above-mentioned problem, a repairing method that includes the following steps is proposed by Japanese Patent Application Laid-Open No. H9-110596. First, removing the defective portion from the base material so that the surface of the base material orients to a direction of preferential crystal growth of the base material; second, carrying out repairing steps by adding a filler material, irradiating the portion to be repaired with a laser beam at a relatively low output power density, so that a diameter of the laser beam becomes relatively smaller, irradiating relatively longer time, and creating a molten pool the ratio of depth to width thereof is small.
However, the above-mentioned method requires removing a broad area of the base material for repairing, and takes a long time for melting, which is inefficient. Furthermore, the direction of the crystal growth in lateral ends of the molten pool is quite different from other parts of the molten pool, which is a problem.
As a method of forming a single crystal build-up portion on a base material, as shown in FIG. 21, there is a method of building up the build-up portion by a half-overlapping method. However, this method has problems of causing uneven distribution of temperature and a filler material, and causing other crystals (different crystals) because it is impossible to control the temperature distribution suitable for controlling the crystalline structure.